6.5 Rem Mag: no loads for 140 gr, Sierra says they did not include them because the OAL would be too long for a short action with heavy bullets. Published 120 gr loads are (surprisingly)~300 fps slower than 6.5-06 or 6.5-284.
My Horrnady manual does lists 140 gr loads up to 2900 FPS.

6.5 Creedmoore: Not listed in any of my loading books. Hodgdon's web site lists 140 gr loads in the 2600 range for max loads.

A practical difference between the 260 Rem and the 264 Win Mag, in a 10 MPH full value wind, you will have about 4" less wind at 600 yards (4.5MOA/28.5" @ 2700 FPS vs 3.8MOA/24.2" @ 3000FPS) The Creedmoore would be a little worse, but not much.

A High master Long Range shooter could probably tell the difference, but most people couldn't. A deer certainly wouldn't.